Remove usage of deprecated protobuf method (protobuf::compiler::java::ClassName) in java_generator.cpp

[sophiechang]

What version of gRPC-Java are you using?

HEAD

What is your environment?

N/A

What did you expect to see?

Deprecated function not used

See https://github.com/protocolbuffers/protobuf/commit/ca4fb2fdab93e15a81602aa0c837c675d32a692e for update

What did you see instead?

Deprecated function used here https://github.com/grpc/grpc-java/blob/master/compiler/src/java_plugin/cpp/java_generator.cpp#L216C10-L216C45

Steps to reproduce the bug

Compile and treat deprecated-declarations as error

[kannanjgithub]

This can be done once we have upgraded past Protobuf major version 33 which introduced QualifiedClassName.